Generally speaking, any one-dimensional resonator optical waveguide is not robust or robust enough against disorders or perturbations caused by defects in its corresponding wafer, defects due to fabrication errors, and/or defects due to degradation over time. In the case of fabrication errors, such errors can include irregularities in the fabrication of the resonators, for instance, resulting in resonators in the row with non-uniform dimensions. Such defects can be problematic in that they can deflect or degrade the quality of light passing through the waveguide. Consequentially, the light output signal can have unwanted modulation in the output signal (i.e., noise) at best and can be undetectable in a worst-case scenario, thereby reducing or blocking transmission of information.
In the case of resonators in the form of silicon-, sapphire-, GaAr-, or silicon-insulator-based micro-rings, for instance, fabrication errors can cause a change in width, length, height, and/or surface roughness, which can lead to resonance mismatch between adjacent resonator micro-rings. An example of a situation where a defect may occur over time is where the chip in which the waveguide resides is subjected to a temperature gradient for a prolonged period of time (i.e., one portion of the chip has a temperature different from another portion). Impurities in the substance (e.g., silicon) in which the resonators are formed can also lead to disorders or perturbations.
In the case of a one-dimensional coupled resonator optical waveguide (CROW) used as a photon delay device, any of the aforementioned defects can be particularly problematic. One-dimensional photonic delay devices can be constructed from a single row of resonators, such as micro-rings or micro-racetracks. A defect in any one resonator, any waveguide between the resonators, or an accumulation of defects along the row of resonators can degrade or block transmission of photons through the waveguide. For instance, the length of delay for a delay device can be given by the size of the array or the length of the photon's path. As the number of resonators and optical features is increased to accommodate longer delays, inherent defects along the photon path can eventually cause a roadblock for the photons. As an example, for a fixed disorder strength (e.g., a fixed disorder), the waveguide may be operational (i.e., allow sufficient passage of photons) with ten (10) series-connected resonators, but may not be operational with one hundred (100) series-connected resonators. The shorter transmission path may have noise, but the longer transmission path is blocked.
FIG. 1 is an illustration of a one-dimensional array 100 of coupled resonators. The array can be used as a waveguide in an optical delay circuit.
The one-dimensional array 100 can include a plurality of resonators, micro-rings 106 in this case, that are serially arranged on a substrate 102. Though FIG. 1 shows seven (7) micro-rings, any suitable number of micro-rings may be implemented, of course, taking into consideration losses and accumulating defects.
Generally speaking, the micro-rings 106 may be formed in a surface of the substrate 102 and can be overlain with a cladding (not explicitly shown). A light source 104 to the array is arranged adjacent a micro-ring 106 at one end of the array. The light source to the array can be a waveguide (e.g., a thin ridge formed in a silicon surface), in which light signals can propagate from a light generation means, such as a laser or light-emitting diode (LED). Light can travel through the light source 104 and can be evanescently side-coupled to the first micro-ring 106 in the array depending upon the frequency of the light signals and the corresponding structure of the first micro-ring 106 in the array. The light signals can be propagated through the micro-rings 106 in the array through direct coupling of adjacent micro-rings 106 as shown by the arrows in FIG. 1, in alternating fashion. The light signals may be delayed as they travel around the micro-rings and from micro-ring to micro-ring. The light signals may be returned to light source 104 and output, or they may be output (via evanescent coupling) at the other side of the array via a light outputting means, such as a waveguide 105.